A bipolar transistor usually includes, for example, an emitter of generally n-doped semiconductor material, a base of generally p-doped semiconductor material, and a collector of generally n-doped semiconductor material. Among various elements and/or parts of a bipolar transistor is an intrinsic base where, during operation, most of the electric current flow through junctions formed by different types of materials. Semiconductor materials inside a bipolar transistor are usually in a crystalline form. That is, atom arrangement of the semiconductor materials generally forms a continuous lattice characterized by a lattice constant.
A Heterojunction Bipolar Transistor (HBT) normally refers to a bipolar transistor wherein a plurality of semiconductor elements such as, for example, Si and Ge are juxtaposed in the intrinsic base of the device to form, for example, a SiGe HBT. In addition to SiGe HBT, other well known HBT transistors may include, for example, AlGaAs/GaAs HBT and InP HBT. Materials inside a HBT transistor are normally arranged to take advantage of increased charge carrier mobility and quasi-static electrical field in the intrinsic base region. Because HBT transistors generally cause smaller delay in signal propagation, measured by a RC time constant, and higher oscillation or cutoff frequencies, they are favored over metal-oxide-semiconductor (MOS) transistors, particularly in high frequency electronic circuit applications, for high-end communication and radar equipment.
Recent HBT devices are usually formed vertically. For example, a HBT may have an emitter formed at the surface of a semiconductor substrate, an intrinsic base or intrinsic base region formed underneath the emitter, and a collector formed underneath the intrinsic base toward the bottom of the substrate. This configuration may be advantageous in forming a thin intrinsic base layer, which is known to be critical for the electrical performance of the device. In addition, the intrinsic base may extend laterally along the semiconductor surface to reach a metal contact on the substrate surface next to the emitter. This lateral extension region, between the intrinsic base and the metal contact, is generally referred to as an extrinsic base.
As is well known in the art, chemical elements and their relative ratios in forming the intrinsic base of a HBT transistor or HBT device are often carefully selected because each of these periodic table elements has a unique lattice constant. A large difference in lattice constant between juxtaposed materials may cause strain and/or stress in the lattice which, if sufficiently large, may ultimately lead to crystal dislocation and cause poor device performance.
The intrinsic base of a HBT transistor or HBT device is normally formed of semiconductor material in crystalline form. Materials forming the intrinsic base are usually deposited through, for example, a Chemical Vapor Deposition (CVD) process. Under controlled process conditions, within lattice constant constraints, and when being deposited over a crystalline substrate (such as the collector), the deposited semiconductor materials may be in crystalline form as well. In general, semiconductor materials of single crystal are more favorable than their poly-crystalline counterpart due to their advantageous electrical behavior.
It is also known in the art that it is advantageous to reduce the dimension of the extrinsic base, of a HBT transistor or device, which is in contact with the collector, in order to reduce parasitic capacitance that may cause RC delay. In a vertical bipolar transistor, one widely used technique toward achieving this goal, while still providing a lateral extension of the intrinsic base, is to introduce an oxide region, such as a Shallow Trench Isolation (STI) region, in-between the extrinsic base and the collector contact in the semiconductor substrate.
Semiconductor materials, such as silicon (Si), deposited over a dielectric material, such as oxide and/or nitride, usually form an amorphous arrangement or poly-crystalline, such as poly-silicon. Therefore, a HBT transistor may include a poly-silicon base (extrinsic base) over the STI region and a single crystalline base (intrinsic base) over the collector region. However, an extrinsic base is not exclusively formed of poly-crystalline. More generally, an extrinsic base may be formed of, in one or more sections or segments, either single crystal, poly-crystalline, or a combination of both.
The continuous improvement in semiconductor device performance has come to the limits that device performance are more and more dependent on the quality of contacts made by the device to the exterior, i.e., the connection of metal lines that link the semiconductor device to the outside world. For example, when a metal contact makes connection to a semiconductor device, there is a phase transition from metal to semiconductor material (such as silicon). On a microscopic level, this transition may lead to certain inherent physical property changes such as the formation of an energy barrier that may affect the flow of electrons during the device operation. Consequently, such barrier may result in loss of conductivity or introduction or increase of resistance of the contact.
Since these physical property changes are setting the limits to the device performance of an HBT device, it is desirable to reduce or, if possible, eliminate any potential detrimental impact, such as dramatic increase in resistance of a contact, caused by physical property changes at the phase transition between a semiconductor material and the metal of contact. In other word, there exists a need in the current art to overcome the deficiencies and limitations described hereinabove.